Such de-icing/anti-icing equipment is widely known from e.g. the patent literature and also from U.S. Pat. No. 4,565,321 “Vehicle for de-icing aircraft” belonging to the assignee of the present application.
The basic layout of the equipment comprises e.g. a vehicle frame or truck and a lever or boom extending from the vehicle and carrying an operator's platform from where the spraying onto an aircraft is provided.
In this technical field it is known to regulate the flow of liquid by regulating the pump motor and EP 0 876 842 B1 “Mixing apparatus for spraying a liquid mixture” also in the name of the present assignee describes the use of two tanks containing liquids to be mixed before spraying, wherein a motor-regulating system is provided for maintaining a required mixing ratio.
In general, such equipment will comprise a number of spraying components including tanks with de-icing and anti-icing fluid, fluid pumps with drive motors, and hoses or pipelines between the pumps and the spray nozzle to pump the fluid from the tank to the spray nozzle at the platform.
In prior art equipment it is common standard to maintain a constant flow rate when anti-icing fluid is sprayed, in order to achieve a constant pressure drop from the fluid pump to the fully opened nozzle, However, this results in too much fluid being used, because the flow from the nozzle cannot be adjusted according to the actual needs.
For anti-icing purposes the fluid will have added thickeners in order to hold a certain fluid-layer thickness on the aircraft surface for an extended period (holdover time) after application. The resulting increased viscosity makes the fluid sensitive to shearing damages, when higher pressure drops over the nozzle occur because the fluid will be accelerated very much and the molecular structure would be stretched beyond cohesion limits.
The occurrence of lower pressure drops over the nozzle would mean too short spray distance of the fluid jet to obtain a satisfactory distribution of fluid on the surface of the aircraft.